Evolution in the genus Arum: a comparative analysis of morpohological and genetic variation.

Abstract

Testing the correlation of morphological and genetic marker variation enables the investigation of evolutionary processes. Knowledge of evolutionary processes can be used to identify those morphological characters that could be used to produce evolutionary meaningful taxonomies. This thesis aims to test the correlation between morphological and genetic marker variation to further understand the evolution of species within the genus Arum and identify those morphological characters that correspond with evolutionary groups. The investigation is carried out at the intraspecific level, intrageneric level and in a putative hybrid zone. At the intraspecific level, genetic (ISSR) and morphological variation was quantified in populations of the morphologically similar species A. maculatum and A. italicum. Populations of A. maculatum showed evidence of isolation by distance, presumably a result of pollinator behaviour and seed dispersal. Leaf patterning in A. maculatum did not correspond to evolutionary lineages. However, similar leaf patterning characters in A. italicum are used to classify the two subspecies neglectum and italicum and the ISSR analysis confirmed that these taxa are genetically distinct. These two subspecies were shown to be interbreeding in sympatric populations. The interbreeding has created a morphological and genetic difference between subsp. neglectum in sympatric populations compared with allopatric populations. At the intrageneric level, a phylogenetic analysis of Arum (using trnL and ITS 1 sequences) indicated that both vegetative and reproductive characters are convergent within the genus. The apparent convergent evolution of reproductive and vegetative characters indicates that both have been important during the diversification of the genus. These convergent characters are not useful for producing classifications that reflect evolutionary groups as the groups they produce are polyphyletic. In the putative hybrid zone, ISSR markers confirmed the presence of A. creticum and A. idaeum hybrids. There appears to be introgression of the A. idaeum genome into A. creticum; this could have implications for the future genetic integrity of A. creticum. Within this hybrid zone, continuous characters were found to be representative of genetic variation, however categorical characters were not. In conclusion, this thesis has shown that even within a single genus, the correlation between morphological and genetic marker variation is influenced by both the taxa being studied and the nature of the morphological trait. In particular, if morphological characters are found to be adaptively important, their correspondence to genetic groups should be tested before their use in taxonomies. The findings of this thesis also suggest there is great value in the complementary use of genetic and morphological analysis for taxonomic studies as well as evolutionary studies. For example, the importance of reproductive characters in the diversification of Arum species has produced a wide range of morphological variation, with limited taxonomic utility due to a tendency for homoplasy. Vegetative characters were also found to need careful testing before use in taxonomies as leaf patterning was found to correspond to sub-species status for one species of Arum but not another. Finally, this thesis has shown that, if closely related taxa are hybridising, variation of continuous reproductive characters may be used as an indicator of hybridisation, even if the morphological characters are potentially polygenic.